The adapter plate

Fabricated by SJ Precision
Material cost $130
Labor cost $350 (CAD data was provided to SJ precision)

As the name implies, the adapter plate "adapts" the motor mounting and centering means to those of the clutch bell. The design must consider the mechanical strength requirements and the distance between the motor flange and machined surface of the clutch bell. That in turn depends on the shaft coupler length dictated by the shaft itself.

I designed the adapter plate myself since it is part of the fun building an EV and is not really difficult. To do that you have to know some dimensions on existing transmission and the motor. Triple check your measurements and your math. Get good drafting software (CAD) allowing drawing in layers and grouping objects. The best way to design the plate so that all the components are match each other is to draw the cross section of the clutch assembly, the motor flange, motor shaft and the coupler in actual size. I use Malz++Kassner CAD 4.7 Workstation for drafting mainly because it is very fast, accurate, allows using digitizer and works with metric as well as English units. If you choose to make an adapter plate yourself, below are the main steps as an example how I did it.

Aluminum slab as sold by supplier to become an adapter plate
The plate after milling, motor side. The visible serpentine pattern is the path of the milling tool. Nice!
Other view angle
Close up of the bolt head recess.
Transmission side, note recesses made for the motor bolt heads
The heads themselves had to be ground off a bit not to interfere with the machined surface of the bell
Do the bolt heads stick out too much? No.
All the bolts are inserted to verify alignment. Perfect!
The plate placed on top of the transmission clutch bell.

You will need to determine the distance between them and between the center of each pin and the center of the transmission shaft. Unless you have that data, start with accurate measurements using CNC equipment. The engine is usually centered in the clutch bell with two dowel pins. +/-0.05mm measurement accuracy is preferred; consider worse than +/-0.1 mm accuracy unacceptable. The shaft of the transmission should not have side play, else replace it's pilot bearing.

External contour of the plate should follow the contour of the clutch bell except the places where the motor flange sticks outside the bell - in such places the plate contour follows the motor flange. The contour accuracy of course is not critical. My motor is quite large and its mounting holes overlap the bell machined surface and its mounting holes. In return, the motor flange blocks access to the bolts tightening the plate to the bell. Oh, well it will be fun assembling such a beast.

Once you determine all the mounting holes location, draw it in CAD and print out in actual size. The fastest way to draw accurately enough the bell shape is this: place flat sheet of drafting the paper on the transmission bell and press against edges with your palm. When removed, the impression of the bell contour with all the mounting and centering holes will be well visible. Outline it with black marker and scan it in. If you don't have access to a large scanner there are two choices - best is to draw a grid so when you cut the paper into smaller pieces which can fit on the scanner bed, you can easily stitch back together pieces of the scanned images. In worst case, just take a photo of the  paper drawing with a digital camera and scale up resulting image to the actual size. Do the same with the motor flange. It is easier to fix any mistakes on the paper now than in the metal later. To determine mounting holes pattern a digital or dial type calipers are sufficient; small misalignment here does not impact anything.

Once you drew all the transmission components, group the lines representing each component to make it a separate object (like flywheel or coupler), assign these objects to different layers to be able to move it around or hide it away to reduce the clutter. "Move" (in CAD) the clutch assembly (flywheel and the pressure plate) onto the tranny splined shaft. It should sit at the same place as in ICE version.  Then move coaxially the motor as close as you can to the clutch shaft, so the shafts almost touch each other. If the distance between tranny flange and the motor flange becomes less than 10 mm, don't move it any closer; the plate must have enough mechanical strength.   In my case I chose the gap between the shafts 0.5 mm. (If the shaft has a threaded hole in its center for the bolt preventing the coupler from sliding off, allow for the bolt head thickness). Then I just measured the resulted distance between the motor flange and transmission flange which became the required thickness of the adapter plate. Turn out to be 18.5 mm. Thick, but not bad. Now you can draw the shaft coupler: since the the distance between the shafts is fixed, the length of the coupler is pretty much predetermined too.

The CAD should allow grouping and draw in layers, this greatly simplifies design.
You will need to measure distances A, B and C on CNC table.
This is how all the components will mate together and how the centering of the plate to the transmission plate is (usually) done.
Another view of the A, B, C measurement. Adapter plate (green) and the motor flange (red) are overlaid.
Cross sectionside view and front view of the drive train components

Since the motor has a round centering recess in its flange, the plate must have respective centering ring sticking out for some height. For the strength and integrity I didn't want to bolt separate centering ring to the plate; rather machine it all from one solid piece of aluminum. Since I must use CNC mill anyway I can mill this round ring on it as well, so no large lathe will be needed. The thickness of the initial plate material must include the ring height. Recess on my motor flange is 10 mm deep, so for, say, 3 mm high ring the total initial thickness of the aluminum slab must be at least 18.5+3=21.5mm. The maximum thickness would be 18.5+10=28.5 mm. (The max of course can be more, that just means I will waste aluminum by initially milling all the surface down to 28.5mm). Obvious choice was standard 25.4 mm (1 inch) thick slab resulting in 25.4-18.5=6.9 mm high centering ring. Cool.

Well, once the plate is drawn it can be machined. In my case a few milling tools took care of everything: centering ring, drilled holes, recesses for the bolt heads. You can see the serpentine trace of the end mill tool as it went around and took off extra material. It is really fun to watch CNC equipment in action! When the plate was ready I've tried to put respective bolts in it and see how well their locations match the mounting holes in the flange. Not perfect, but it's OK since the only function of the bolts is to keep the sandwich together; dowel pins placed with great precision keep it all centered and prevent the shear movement.

If you wish to examine the drawing of my drive train in CAD, download and install free viewer from the original Malz++Kasha web site or its copy from my site. The drawing file is here (Shift+click to download).

The shaft coupler

Fabricated by American Machine and Gear
Material and labor cost $300

Another critical piece of the drive train is the shaft coupler. This part is installed onto the shaft of the motor, in my case it has machined splines inside matching the motor's splines. Once on the shaft it is set against the ring on the shaft and held from sliding off by special bolt. The bolt goes into the threaded center hole of the shaft. Since there is no axial force onto the coupler other than the force created by the spinning mass when the vehicle turn (for the cars with the motor across the body arrangement), this bolt does not have to be very strong. Mine is machined of stainless steel with M6 thread. Special tool was made (out of the large bolt head) to tighten this bolt and thread locking compound ("LockTite") was used on the thread as well as on the splines. After tightening it becomes part of the motor shaft. Beautiful!

The shaft coupler. This side is facing the motor flange.
The other side if the shaft coupler facing flywheel. Note recess made for the special bolt.
Special bolt (thread M6) and the tightening tool made from a hex head of the large bolt.
Coupler ready to be installed onto the motor shaft.
Done. Isn't it cool?